Modelling, Measurement and Interpretation of Space Charge Accumulation in Polyethylene Insulation for High Voltage DC Applications

The formation of space charge inside insulating materials causes non-uniformity of the internal electric field leading to overstress of the insulation material and reduced lifetime. The pulsed electro-acoustic (PEA) method is an important tool to determine space charge distributions in insulating materials. This work proposes a novel approach for the interpretation of raw PEA data that offers a more robust and objective method for the determination of space charge and electric field profiles than traditional space charge recovery techniques proposed in the literature. This work integrates an electro-thermal charge transport model incorporating electric field, temperature and impurity dependent electrical conductivity with a PEA simulation model to provide simulated raw PEA output data. The electro-thermal charge transport model calculates the time dependent space charge accumulation in insulating materials whilst the PEA simulation takes the space charge profile at a given time and calculates the raw PEA output waveform. The dynamics of space charge accumulation and decay under DC and DC containing an AC ripple voltage were then studied using single and double layer samples and under isothermal and temperature gradient conditions. In the case of low density polyethylene (LDPE), the electrical conductivity and the resultant space charge accumulation was controlled by impurities. Impurities could be removed by thermal conditioning under vacuum at elevated temperatures but a temperature in excess of 50C runs the risk of thermal ageing and increased electrical conductivity. The rate of decay of space charge following voltage removal was found to be much lower than during charging owing to the field dependent electrical conductivity. Under thermal gradient conditions the combined model gave superior fits to the raw PEA data than were obtained using a bipolar transport model. The effect of the ripple voltage on the DC voltage was to enhance space charge accumulation and therefore enhance electro-thermal ageing.